The invention relates generally to the operation of nodes within telecommunication networks. Specifically, the invention provides a method of substantially reducing synchronization rearrangements in nodes within synchronous transmission systems such as SONET networks.
SONET (Synchronous Optical Network) has become a popular standard for optical transport of telecommunications traffic. Details of the SONET standard can be found in the BellCore document GR-253-CORE SONET Transport System: Common Criteria. Those skilled in the art are familiar with the use of SONET and its inherent advantages over asynchronous transport technologies. Pertinent to the present invention is the synchronization of SONET networks.
A SONET network is a collection of nodes, or network elements, connected by optical fiber. Nodes within SONET networks can be connected in numerous ways, including in both linear and ring configurations. In a SONET network, each network element must be clocked at the same or nearly the same rate in order to facilitate data transmission at multiples of the basic STS-1 line rate of 51.84 million bits per second (Mbps). The clocking signal utilized at a given network element can be received from a number of sources such as: an internal clock, a signal from another network element in the system, or an externally timed clock such as a building integrated timing supply (BITS). These timing sources are categorized by the level of quality of the timing signal and arranged in a hierarchy where a Stratum 1 (ST1) timing source, such as a Cesium atomic clock, is considered the most stable and accurate while a SONET Minimum Clock (SMC) is the least stable timing source allowed for use in a SONET system under the present standards.
Nodes, or network elements, within SONET systems are configured generally to utilize and synchronize to the best, or highest quality timing source available to it. A common design of a SONET network has one or more nodes externally timed with one or more ST1 and/or ST2 source(s). The other network nodes may be line timed, thus receiving their timing signals from the externally timed nodes over one or more paths. In such a design, each of the line timed nodes must choose the best signal from among the timing signals supplied by the externally timed nodes, either directly or through intervening nodes and its own internal clock and any locally connected external clocks. When a node determines that the best timing signal available to it is better in quality than the signal to which it is currently synchronized, the active reference signal, a synchronization rearrangement occurs.
The inventors have found that under certain circumstances, when there is a change in the timing signal utilized by one node in a network, considerable delays are caused to occur in one or more of the other nodes in the network which can negatively affect the entire flow of traffic. After an extensive investigation, the inventors have determined that under certain conditions, some network elements are susceptible to being placed in states of synchronization switch oscillation through the performance of repeated and unnecessary synchronization rearrangements. This can occur, in a ring configuration, when a line timed node receives conflicting information from the two sides of the ring regarding the quality of the timing source from a first node shortly after a change in the timing signal utilized by the first node. In such a case, the inventors found that the line timed node was receiving an updated synchronization signal quality level message from one side of the ring at a finite amount of time before it was receiving a similar updated message from the other side of the ring. This apparent difference in timing signal quality, although not a real one, sometimes caused a given node to perform one or more unnecessary synchronization rearrangements. Similar yet unnecessary synchronization rearrangements can occur at a line-timed node in a linear configuration as well as a ring configuration. This can occur when a common timing source, connected indirectly to multiple nodes, fails and updated synchronization status messages arrive at an interconnected line-timed node at different times. The cause for the time delay, as determined by the inventors, can be attributed to the propagation delay due to differences in line length and equipment dependent delay in propagating the synchronization signal quality level message over nodes of differing numbers and types.